1. Field of the Invention
The present invention relates generally to thermal printing apparatus and, more specifically, to a thermal transfer printing system for producing a printed image of a recorded signal by use of selectively energized heating elements and an ink ribbon having a sublimable dye.
2. Description of the Prior Art
Following use of a video camera, such as the known electronic still camera that is capable of recording signals representing an image or other video information on a magnetic sheet or disc, it is generally desirable to obtain a hard copy or print of the recorded video information. One proposed system for supplying a hard copy employs a thermal transfer printing system wherein an ink ribbon having a sublimable dye is employed. Such proposed thermal printing system typically also uses a rotatable platen and a thermal print head, with the paper being wound around the platen and rotated in a step-by-step fashion in front of the thermal print head that is connected to receive the recorded image signals. The ink ribbon include three, separately arranged, color-dye areas, and each color is completely printed onto the paper in succession. Additionally, a black-dye area is employed to highlight and add definition to the printed color image. The heat produced by the energization of the individual heating elements forming the thermal print head causes the sublimable dye coated onto the ink ribbon to be thermally transferred to the printing paper arranged around the platen.
The thermal print head is typically formed of up to 500 heating elements, each of which corresponds to a numbered dot forming one vertical line of the picture image. The heating elements comprise a plurality of individual resistive elements each having a separate input lead connected to be driven by a pulse signal to generate the heat. The video signals typically are modulated in pulse width form and fed to the respective heating elements utilizing overall system timing. By using pulse width modulation, if the amplitude of the color signal is large then the heating element is driven for a longer duration of time, so that the thermally transferred amount of sublimable dye is increased, thereby to provide a greater printing density.
In the thermal printing head the resistive heat elements are horizontally aligned and due to manufacturing tolerances some deviation in the actual resistance value of the heating elements will always be present. Because the heat generated will be correspondingly varied because of the resistive variations, the density of the resultant print will also have undesirable variations. There have been several techniques proposed to prevent these variations in the density of the resultant print, such as employing a separate power source for each of the heating elements forming the thermal head. Another proposed approach was to provide an individual balancing resistor for each of the heating elements in the head and still another technique proposed to adjust the electrical power applied to each of the resistive elements following production of an unacceptable print. In all of these previously proposed approaches, the technique of controlling the electric power consumption has proven to be the most favorable in compensating for fluctuations of the resistance value caused by manufacturing tolerance deviations. Nevertheless, in all of the suggested approaches a separate, individual circuit for each head element is required, so that the resultant circuit arrangement for the thermal printing system becomes overly complex and expensive because of the high fabrication costs.